This invention relates to the metallothermy and continuous drawing off of metals or alloys, in cold inductive crucibles. The metals or alloys are produced from metallic oxides or salts. The invention particularly concerns the preparation of uranium metal from a uranium oxide or salt. It also concerns the preparation of an alloy of uranium or another metal.
State of Prior Art
Conventionally, uranium metal is obtained after transformation of UO2 oxide into UF4, followed by a step to reduce the tetrafluoride into uranium metal. No current industrial process is capable of directly producing solid uranium metal ingot from the dioxide. The action of a reducing metal such as Ca or Mg on uranium dioxide, despite its very strong exothermal nature, cannot melt the lime or magnesia, and uranium metal is obtained divided in an unmeltable solid. The slag may be dissolved in an acid, but the processing cost is prohibitive for large scale production and there is a risk that the divided metal could be attacked. On the other hand, the slag composed of fluorite or sellaite formed during reduction of UF4 by these metals is relatively meltable. The uranium assembles in situ in the form of an ingot. Slag generates a solid waste, treated by a wet method before being permanently stored in a controlled tip.
At the present time, constraints necessary for the reduction of waste involve the use of recycling processes, for example using electrolysis.
Patent U.S. Pat. No. 5,290,337 divulges a process for the reduction of uranium oxide by magnesium capable of recovering uranium metal and regenerating the metallic reduction agent. Uranium oxide (for example UO2) is reduced in the presence of molten salts, for example MgCl2/MgF2 or MgCl2/NdCl3 in order to improve the settlement of uranium metal, as recommended by C. MORANVILLE and J. DUBUISSON in Le Nouveau Traitxc3xa9 de Chimie Minxc3xa9rale (The New Treatise of Inorganic Chemistry) published under the management of P. PASCAL, Volume XV, book 1, 1960, Masson and Company, page 187, and J. H. BUDDERY in Metallurgy and Fuels, 1956, vol. 4, pages 24-32. The metal is recovered in a layer of molten ZnMg or CuMg alloy. UO2 is added into the two-phase medium composed of the molten alloy (ZnMg) and the mix of molten salts (MgCl2/NdCl3), stirred and heated to 750xc2x0 C. Some of the magnesium reduces UO2 by forming U and MgO. The MgO then reacts with neodymium chloride to produce magnesium chloride and neodymium oxide. The uranium metal is incorporated in the ZnMg phase. When the reactional medium is no longer stirred, the phases separate. The denser metallic phase settles at the bottom of the crucible. After cooling and solidification, the phases are mechanically separated. The uranium metal is then separated by evaporation of ZnMg. Mg is regenerated by electrolysis of the salt.
Thus according to U.S. Pat. No. 5,290,337, reduction of UO2 by Mg takes place in a two-phase mix while being stirred. The principle of two separate phases only appears after the end of the process that can only be a discontinuous system (xe2x80x9cbatch processxe2x80x9d). Therefore, this process has the disadvantage that it cannot directly and continuously produce uranium metal at the output from the crucible. Two additional steps are necessary to separate the constituents, firstly a mechanical separation of the phases and then a distillation. Furthermore, the bath is necessarily remelted before electrolysis and regeneration of Mg, and it cannot subsequently be recycled unless an additional step is added to convert the neodymium oxide into chloride.
Since the 1960s, many studies, publications and patents have been made on continuous melting and drawing off of ingots. The metal is usually added directly into the drawing off crucible, after being produced in other structures, for example by electrolysis, metallothermy or reduction by a metalloid (carbon, sulfur).
The xe2x80x9cEconomically Producing Reactive Metals by Aerosol Reductionxe2x80x9d article by J. D. LELAND, published in the J.O.M. review, pages 52-55, October 1996, divulges a process for obtaining titanium, hafnium or zirconium semi-continuously from their precursor salts (chlorides). The reduction process by aerosols consists of a reaction between two jets of products.
The reducing metal (for example Na or Mg) is in the form of an aerosol between 400 and 600xc2x0 C., and the metal chloride to be reduced is in vapor form. Due to the exothermal nature of the reaction, the chloride formed is vaporized whereas the solid metal drops to the bottom of a liquid metal bath. The temperature of the medium is stabilized at about the boiling point of the salt formed, typically 1100-1200xc2x0 C. The metal is collected and molten starting from the bottom of the bath in an inductive cold crucible. This process has two main disadvantages, firstly the necessity to have an exothermal reaction in order to vaporize the slag and the initial product that has not reacted, and the difficulty in controlling feed flows. Furthermore, this process would not be applicable in the case of exothermal reactions in which there is a risk of reaching the thermodynamic inversion temperature or if the slag boiling temperature exceeds the thermodynamic inversion temperature.
The invention overcomes the disadvantages of prior art mentioned above by proposing a solution that enables complete and continuous production as far as the preformed metal, starting from the metal salt or oxide.
Therefore, the purpose of the invention is a metallothermy and continuous drawing off process for a metal product, composed of at least one metal, comprising:
a metal production step in a first cold crucible heated by induction in which reduction of the oxide or salt of the said metal is provoked in a reducing medium composed of a floating layer of melting material and in which the formed metal settles in a solvent medium consisting of a bath composed of at least one molten salt that absorbs the slag resulting from the reduction reaction, the maximum input flow of the floating layer of oxide or salt of the said metal being determined by the thickness of the floating layer and the temperature of its top surface such that the reduction of the oxide or salt of the said metal takes place entirely in the floating layer;
a step in which the settled metal is collected and melted, that takes place in a second cold crucible heated by induction located under the first cold crucible, in order to enable continuous drawing off of the metallic product.
This determination of the maximum feed flow may depend on test results and/or a model to produce nomograms. The floating reducing medium can be used to perfectly control the temperature and avoid reversibility of the reactions.
Induction heating of the first crucible and the second crucible can take place at different frequencies.
The reducing medium may include a material chosen among a metal, a mix of metals, a metalloid or a mix of metalloids.
The oxide or the salt of the said metal to be produced may be added into the first cold crucible above the layer if this oxide or salt is in the solid state or the liquid state. The oxide or salt of the said metal to be produced may be added into the first cold crucible under the layer if this oxide or salt is in the gaseous state.
If the metallic product to be produced is an alloy of at least two metals, the first cold crucible may be fed by a mix of oxide(s) or salt(s) of these two metals. As a variant, one of these metals may be added into the first cold crucible directly in its metallic form, the other of these metals being added in the form of an oxide or salt.
Advantageously, the process also comprises a step in which the reducing medium is regenerated by electrolysis of the slag present in the solvent medium. The solvent medium is drawn off continuously.
Another purpose of the invention is a device for embodiment of the process described above, characterized in that it comprises:
a first crucible with cold walls equipped with induction heating means and means of adding the oxide or salt of the said metal to be produced into the reducing medium.
a second crucible with cold walls equipped with induction heating means located under the first crucible, the upper part of the second crucible communicating with the lower part of the first crucible, the lower part of the second crucible being provided with means of continuously draining the metallic product.
Preferably, this first crucible has a larger cross sectional area than the second crucible. In this case, the junction between the two crucibles may be formed by a board of refractory material, for example a ceramic board.
Preferably, the induction heating means are located on the first and second crucibles such that it forms a settlement cone composed of the solid solvent medium, under the influence of the cold walls of the crucibles.
Advantageously, the first crucible is equipped with means of drawing off the solvent medium, operating continuously. It is also equipped with means of adding the reducing medium, operating continuously. It is also equipped with means of adding the oxide or salt of the said metal to be produced, operating continuously.
The process according to this invention is particularly applicable to obtaining uranium metal.
If the uranium compound that is reduced is UO2 oxide or U3O8 oxide, the reducing medium may consist of lithium, and the solvent medium may comprise at least one of the following salts: LiCl, KCl, BaCl2, LiF, CaF2 and BaF2. If the uranium compound that is reduced is UF4, the reducing medium may be composed of a metal chosen among Ca, Mg, Li, K, Na or a Ca-Mg mix, the solvent medium may comprise at least one of the following salts: MgF2, MgCl2, LiCl, KCl, BaCl2, LiF, KF, CaCl2, CaF2, NaF, NaCl and BaF2. If the uranium compound that is reduced is a double fluoride of uranium and an alkali or alkali earth element, the reducing medium may be a metal chosen among Ca, Mg, Li, Na or K, or a mix of at least two of these reducing metals, and the solvent medium may include at least one of the following salts: MgF2, MgCl2, LiCl, KCl, NaCl, NaF, BaCl2, LIF, CaF2, CaCl2 and BaF2. If the uranium compound that is reduced is Cs2Ucl6, the reducing medium may be a metal chosen among Ca, Mg or Li or a mix of at least two of these reducing metals, the solvent medium may comprise at least one of the following salts: LiCl, KCl, CsCl, BaCl2, LiF, CaF2, BaF2, MgF2 and MgCl2.